Spacewars is a project mainly written in Java, it's free.
A multiplayer, networked version of spacewars
Call ant (without arguments to compile) ant run to run a copy of the project java -jar dist/spacewars.jar to run the precompiled version
Controls: A - Rotate CCW D - Rotate CW W - Accelerate Q - Hyperspace to random server R - Respawn (if dead) Space - Fire a missile